Quasars comprise the most luminous subclass of AGNs, with nuclear magnitudes MB < -21.5 + 5 log h0. A small minority (5-10%) of these sources are the strong radio sources that originally defined the quasar class. Quasars are distinguished from Seyfert galaxies in that in general they are spatially unresolved on the Palomar Sky Survey photographs, which means in practice that they have angular sizes smaller than 7 arcsec. Many of these sources, however, are surrounded by a low surface brightness halo (sometimes called ``quasar fuzz''), which does indeed appear to be starlight from the host galaxy, and a few sources have other peculiar morphological features, such as optical jets (e.g., 3C 273). Quasar spectra are remarkably similar to those of Seyfert galaxies, except that (a) stellar absorption features are very weak, if detectable at all, and (b) the narrow lines are generally weaker relative to the broad lines than is the case in Seyfert galaxies. A ``typical'' QSO spectrum, constructed by averaging observations of a large number of QSOs, is shown below.
Figure 2.2. A mean QSO spectrum formed by averaging spectra of over 700 QSOs from the Large Bright Quasar Survey (Francis et al. 1991). Prominent emission lines are indicated. Data courtesy of P. J. Francis and C. B. Foltz.
Adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)